A fiber-reinforced plastic molded article having a closed cross-section is widely used in a range from large molded articles such as the body or wings of an aircraft to small molded articles such as a bicycle frame, a tennis racket, a fishing rod, and a golf shaft. In addition, a fiber-reinforced plastic molded article having an open cross-section is widely used for helmets and the like.
As a core for forming the closed cross-section, a core formed into a predetermined shape by wrapping a group of powder particles and/or a group of grains (hereinafter, a powder grain group) in a packaging film and vacuum-packaging the result, a core using a molded product formed by blow molding, and the like are used. A method for molding a molded product using a core which is made by having a vacuum-packaged powder grain group formed into a predetermined shape is disclosed in, for example, JP 2-238912 A (Patent Document 1). In addition, the core formed by the blow molding is used in a method for manufacturing a multilayer plastic molded product disclosed, for example, in JP 7-100856 (Patent Document 2).
The invention described in Patent Document 1 will be described with reference to FIGS. 12 to 14 as Conventional Example 1 related to the method for molding a fiber-reinforced plastic molded article having a closed cross-section according to the present invention. FIG. 12 illustrates an intermediate state in the process of manufacturing a molded article having a hollow portion which is one type of closed cross-section by using a mold 30. That is, a sheet-like lower fiber-reinforced thermoplastic resin material (hereinafter, referred to as lower FRTP) 34 which has been pre-heated into a molten state is placed on the lower mold 31 of the mold 30. Since the lower FRTP 34 is in the molten state, the lower FRTP 34 is hung and depressed in a concave portion of the lower mold 31 by its own weight.
A core 33 formed by wrapping a powder grain group 33a in a packaging material 33b and solidifying the result into a predetermined shape by vacuum packaging is placed in the concave portion of the lower FRTP 34. On the upper portion of the lower FRTP 34 in which the core 33 is placed, another sheet-like fiber-reinforced thermoplastic resin material (hereinafter, referred to as upper FRTP) 35 which has been heated to be in a molten state is placed. In this state, the periphery of the core 33 is in a state of being enclosed by the lower FRTP 34 and the upper FRTP 35.
The upper mold 32 of the mold 30 is lowered from this state to heat and cure the lower FRTP 34 and the upper FRTP 35 between the upper mold 32 and the lower mold 31 such that the lower FRTP 34 and the upper FRTP 35 are integrally molded in a state of accommodating the core 33 therein. In order to discharge the core 33 from the semi-finished product produced here, one or more small holes which communicate with the inside of the core 33 are bored through the semi-finished product. When the holes are bored through the semi-finished product, air infiltrates into the powder grain group 33a of the vacuum-packaged core 33 and the binding of the powder grain group 33a is loosened.
In addition, at least the powder grain group 33a included in the core 33 is discharged to the outside of the semi-finished product through the holes bored through the semi-finished product, thereby completing a molded article. If the packaging material 33b in which the powder grain group 33a is packaged is made of a material having good releasability from the molded article, the packaging material 33b can be released leaving the hollow portion of the molded article.
The invention described in Patent Document 2 will be described with reference to FIG. 15 as Conventional Example 2 related to the present invention. FIG. 15 illustrates a state where a core 43 molded by blow molding is set between molds 41a and 41b used for forming an outer layer. As illustrated in FIG. 15, the molds 41a and 41b are configured to accommodate the core 43 therein, and when the molds 41a and 41b are closed, a cavity as a hollow portion is formed to be filled with a molten resin between respective molding surfaces 42a and 42b of the molds 41a and 41b and the core 43.
Molten resin 45 which is plasticized by an extruder 44 is supplied into the cavity. By supplying the molten resin 45 into the cavity in the molds 41a and 41b in the closed state, a product having a hollow portion and desired shape can be molded by integrating the core 43 with the molten resin on the core 43. However, in a case where the heat resistance of the core 43 is low compared to the temperature of the molten resin or in a case where the thickness of the core 43 is small, the core 43 may be deformed in the molding process by the pressure applied to the core 43 in the process. In addition, in a case a wide flat part is present as the shape of the core 43, the rigidity of the flat part is likely to be insufficient, and thus the core 43 may be deformed.
In order to prevent the deformation of the core 43, in the invention described in Patent Document 2, a configuration in which the internal pressure of the core 43 can be increased is employed. As a configuration for this, a pressurizing unit 46 which communicates with the inside of the core 43 is provided, and pressurized gas or liquid is introduced into the core 43 from the pressurizing unit 46 to increase the internal pressure of the core 43 and prevent the deformation thereof.
On the other hand, a method for molding a fiber-reinforced plastic molded article having an open cross-section is disclosed in, for example, JP 4118685 B2 (Patent Document 3). FIG. 16 corresponds to a figure attached to Patent Document 3. In the molding method, a mold assembly 57, in which a fiber-reinforced plastic material (composite layup) is disposed between one or more pair(s) of separated mold parts 57a and 57b which form a mold cavity for the fiber-reinforced plastic molded body, is disposed to be interposed between a pair of elastically-deformable chamber walls 55 and 55 which are fixed to the first and the second pressure chambers 51 and 52 to oppose each other. Fluid heated and pressurized to predetermined levels is circulated through the first and second pressure chambers 51 and 52. Each pair of mold parts 57a and 57b respectively correspond to a so-called a pair of male and female molds in typical molding.
The mold assembly 57 is heated and pressurized by the circulating fluid heated and pressurized to required levels via the respective chamber walls 55 and 55 of the first and second pressure chambers 51 and 52. During the heating and pressurizing, the pressure chambers 51 and 52 hold a state in which the elastically-deformable chamber walls 55 and 55 are arranged, and the fiber-reinforced plastic material is compressed via the mold parts 57a and 57b and cured, thereby a fiber-reinforced plastic structure is molded.